In rats, an ED50 for analgesia of morphine, meperidine, viminol R2 or azidomorphine decreases the turnover rate of acetylcholine in cortex and hippocampus but not in striatum. Naltrexone, a narcotic antagonist, reversed the action of the narcotics. These results suggest a correlation between analgesia and decreased turnover rate of acetylcholine in cortex and hippocampus. Although there is no correlation between sites where morphine reduces acetylcholine turnover rate and the density of opiate receptors, the present studies indicate that in some brain structures opiate receptors control the metabolism of acetylcholine.